Behaviour of NiTi SMA Helical Springs under Different Temperatures and Deflections

Shape memory alloys (SMAs) are one of the most widely used smart materials in many applications because of their shape memory effect property. In this work, the behaviour of NiTi SMA helical spring was evaluated through isothermal force-displacement experiment (IFDE) and shape recovery force experiment (SRFE). The transformation temperatures of SMA spring were determined by differential scanning calorimetry (DSC) test. In situ heating of SMA spring by direct electric current was used instead of conventional furnace heating. The continuous measurement of temperature of SMA spring during heating and cooling was ensured with attaching the thermocouple by heat shrinkable sleeve. From IFDE, the force-deflection behaviour under different constant temperatures and from SRFE and the force-temperature behaviour under different constant deflections are obtained. The results of IFDE show that the force increases and the residual displacement decreases with an increase in the temperature, and the stiffness of the spring at austenite state is greater than that at martensitic state. The results of SRFE show that the shape recovery force increases more or less linearly with an increase in the initial deflection for the same temperature range. But the shape recovery forces are not similar during heating and cooling stages. This paper presents the experimental setup, experimental procedures, and the observed behaviour of SMA helical springs under different temperatures and deflections. 1. Introduction A shape memory alloy (SMA) has two unique properties: (1) pseudoelastic effect (PEE) or superelastic effect (SEE) and (2) shape memory effect (SME). These unique properties are achieved through a solid-state phase change (molecular rearrangement) that occurs in an SMA. The phase transition occurs between the high temperature parent phase austenite (A) and the low temperature phase martensite (M) and is referred as martensitic phase transformation. SME refers to the ability of the material, initially deformed in its low temperature phase (M), to recover its original shape upon heating to its high temperature phase (A). Super elasticity occurs when the martensitic phase transformation is stress induced at a constant temperature which is above austenite transformation temperature. SME of the SMAs can be used for actuation, and SE of the SMAs can be used in applications such as vibration isolators and dampers. It is understood from the literature survey that many attempts have been made towards the description of the thermomechanical behavior of SMA wires under various